1. Field of the Invention
This invention relates, in general, to a method and apparatus for extruding plastic materials and, in particular, to a new and useful method and apparatus for making a non-slip plastic film.
2. DESCRIPTION OF THE PRIOR ART
Plastics films, especially waterproof plastics films, such as polythen, which at present are used for making different kinds of wrappings, e.g. sacks for fertilizers and the like, for wrapping timber and as protecting covers for example on construction sites, have the great disadvantage of being smooth and glossy and having a very low friction coefficient. As a result of this e.g. sacks made of such plstics film and piled on top of each other tend to collapse very easily and accidents occur. Also the handling itself is time-consuming. The same applies when this plastics film is used for wrapping timber or as covering material on construction sites, especially in the winter when snow and ice increase the risk of slipping. Piling and handling of the slippery wrapping is further aggravated by snow and ice.
Attempts have been made at reducing the risk of slipping by making the surface rough, e.g. by embossing, but such embossing does not have the desired effect as the embossment under pressure rapidly becomes smooth. It is of course possible to give the surface the desired roughness by laminating the plastics film with some other material but this involves special manufacture which is relatively expensive and accordingly not applicable for cheap dispensable wrapping material.
The most common plastics material for sacks and timber wrapping is the polyethylene based plastics material commercially known as polythen, from which films, sacks and the like are ordinarily prepared by extruding, whereby different methods are applied. According to one method the molten plastics material is fed through a nozzle provided with an annular slit, the axis of which is vertically aligned. Above the nozzle the plastic hose or tube is exposed from the inside to air pressure of some degree, whereby the tube is substantially expanded. The thus formed seamless plastics web of annular cross section is fed in an upward direction until it is cooled and is subsequently folded in a direction transverse to the web, e.g. against a roll and from thereon, usually in a downward direction to be wound onto a reel. The air space is defined on the one hand by the nozzle and on the other hand by the point of folding. By adjusting the air pressure it is possible to vary the film thickness to a certain degree. The film is in this case stretchable both in the longitudinal and the transverse direction.
A plain film may also be prepared by forcing the material through an elongated nozzle against a cooled glossy roll which performs the shaping of the film itself. Stretching thus occurs only in the feeding direction of the material web.
Also further methods exist and the plastics film manufactured becomes in all these cases slippery and glossy due to the structure of the material, which is a great disadvantage of the otherwise usable packing material.